It was a nightmare scenario by any reckoning: workers installing piping at a school accidentally set off a release of gas and ran to warn everyone to evacuate. Some people made it out of the building before a thunderous explosion destroyed it. Others didn’t.

That’s what occurred on the morning of August 2, 2017, on the north campus of Minnehaha Academy, a private school in Minneapolis, Minnesota. The blast killed two employees, custodian John Carlson and receptionist Ruth Berg, and seriously injured nine others.

A brief just released by the National Transportation Safety Board (NTSB) says that at the time of the explosion, two workers were installing piping to support the relocation of gas meters from the basement of the building to the outside. Two new meters mounted on an exterior wall were ready for the piping to be connected. While workers were removing the existing piping, a full-flow natural gas line at pressure was opened. The workers were unable to control the release of the gas; thus, they evacuated the building and warned others to evacuate. The explosion occurred during their evacuation.

The probable cause: the pipefitting crew’s disassembly of piping upstream of a gas service meter. Contributing to the incident was the lack of detailed documentation that clearly established the scope of work to be performed.

News sources report that a number of civil lawsuits have been filed by surviving Minnehaha Academy employees against Master Mechanical and Center Point Energy.